Chop the rosemary leaves finely and put in a bowl with the wine and jelly. Cut garlic clove into two or three chunks and crush each chunk with the flat of the knife's blade, then add to the rest of the marinade ingredients. Stir to combine. If the jelly refuses to dissolve into the wine, zap it in the microwave for a few seconds.
Cool marinade, if needed, put the meat in a large freezer bag and pour in the marinade. Seal the bag with as little air in it as possible and place in the fridge, on a large plate in case of leakage, until a couple of hours before you want to cook it.
At least an hour before you want to cook the meat, take it out of the fridge to come to room temperature.
Preheat the oven to 400°F. Place the lamb in a roasting pan and rub with the oil. Strain the marinade into a jar then pour about a half into the roasting pan; reserve the rest. Add cold water to the tin so that there is about 1/16th inch depth of liquid (ie a thin layer coating the entire bottom). Bake 15 minutes and then reduce the heat to 350°F. Baste with the pan juices and then roast for a further 25-35 minutes, until themometer reads 145°F.
Once the meat is cooked, remove to a warm serving plate and tent with foil to keep it hot whilst you make a gravy. Strain the juices from the roasting pan into a small saucepan add the reserved marinade and bring to the boil, stirring occassionally. If the gravy is very thin, carefully whisk in the cornstarch slurry until thickened, return to a boil while whisking lightly. Strain though a sieve and cheesecloth to remove sucs. Serve the sauce alongside the meat, carved into thick rounds.